There has heretofore been known a radio terminal connectable to multiple networks employing different radio communication schemes (including “1xEV-DO” in compliance with CDMA2000 and “WiMAX” in compliance with IEEE802.16e, for example).
The radio terminal includes a mobility manager (MIH user; Media Independent Handover User) configured to manage mobility between the multiple networks, and multiple link controllers configured to respectively control radio links which are established between its own terminal and the respective networks. Moreover, there is a proposal to provide a handover controller (MIHF; Media Independent Handover Function) which is located between the mobility manager and the multiple link controllers, and which is configured to control handovers between the multiple networks (Non-patent Document 1, for example).
To be more precise, the mobility manager notifies the handover controller of types of multiple link parameters (SINR, RSSI, DRC, Tx_Power, and DRC_Lock, for example) and information (MIH_Configure.request) indicating multiple thresholds set up for the respective link parameters. Here, the thresholds include a first threshold (Initiate Action Threshold) for judging whether or not to perform preparation for a handover (Initiation Action), a second threshold (Execute Action Threshold) for judging whether or not to execute the handover (Execute Action), and a third is threshold (Rollback Action Threshold) for judging whether or not to cancel preparation for the handover (Rollback Action).
The handover controller notifies the link controller, which controls the radio links already established, of information (Link_Configure_Threshold.request) indicating the types of the multiple link parameters and the multiple thresholds set up for each of the link parameters.
The link controller notifies the handover controller of information (Link_Parameters_Report.indication) indicating a link parameter value when the link parameter value becomes worse than the first threshold. The handover controller notifies the mobility manager of information (MIH_Link_Parameters_Report.indication) indicating the link parameter value.
Based on this notice, the mobility manager instructs preparation of a handover, i.e., an establishment of the radio link which is yet to be established.
Subsequently, the link controller notifies the handover controller of the information (Link_Parameters_Report.indication) indicating the link parameter value when the link parameter value becomes worse than the second threshold (a value representing a situation where a radio environment becomes worse than that of the first threshold). The handover controller notifies the mobility manager of the information (MIH_Link_Parameters_Report.indication) indicating the respective link parameter values.
Based on this notice, the mobility manager instructs execution of the handover, i.e., switching of the network to which the radio terminal is connected.
In the meantime, the link controller notifies the handover controller of the information (Link_Parameters_Report.indication) indicating the link parameter value when the link parameter value becomes better than the third threshold. The handover controller instructs cancellation of the preparation for the handover (Initiation Action).
As described above, since the link controller only has to perform threshold judgment, a process load on the link controller is reduced.    Non-patent Document 1: IEEE802.21 Draft Standard (P802.21/D04.00)